"This is the new prototype for a system of rocky planets beyond our own,” Ford said. "It changes our understanding of the frequency of solar systems like our own in deep space.”

The planets orbit Kepler-11, a sun-like star about 2,000 light years away. With temperatures hotter than Venus — likely more than 400 to 1,400 degrees Fahrenheit — the planets range in size from twice to 4 times Earth's diameter. The five confirmed planets are larger in mass but less dense than Earth, and closely packed, taking from 10 to 47 days to orbit the star. There is almost certainly a sixth planet orbiting nearly twice as far away, but its distance from the star makes its confirmation more difficult, Ford said.

Although scientists haven't yet determined the planets' composition, their densities offer some clues. Denser than water but less dense than Earth, "their surfaces could be rocky or a combination of rock and ice, but they also have a lot of gas because their densities are so low,” Ford said.

The Kepler mission is searching for planets in what is known as the habitable zone — where a planet could have liquid water on its surface — using a space telescope staring at one portion of the Milky Way for years on end. The Kepler-11 planets were detected not by direct observation but by tracking the dimming of a star's light when planets pass between the star and the telescope. The objects orbiting Kepler-11 were confirmed as planets by observing small irregularities in the time when each planet transits across the star, known as the transit-time variation method.

The Kepler-11 system marks the second set of planets identified by this technique, which allows scientists to find planets orbiting stars that would otherwise be too faint to be confirmed. The first system discovered by this method was Kepler-9, announced Aug. 26, 2010, which included two gas-giant planets. Kepler-11 is nearly 500 times dimmer than stars that are typically discovered by traditional methods.

"This comes as a surprise to those accustomed to the traditional planet-discovery technique,” Ford said.

Kepler-11 also is remarkable in that the planets travel in nearly the same plane, similar to those in our solar system, making it much more likely that multiple planets could be detected orbiting a single star. The next step will be to delve deeper into the data continuing to arrive from the Kepler spacecraft to determine mass and orbits of the planets more precisely, providing clues to how the planets formed.

"Much of the scientific community thought that multiple planets transiting the same star would be unlikely,” Ford said. "That idea has been completely overturned by this new discovery. Without the transit-timing method, these planets might have gone unconfirmed for years.”